Control system for occupant restraint system

ABSTRACT

A control system for an occupant restraint system including an airbag which is mounted on an automotive vehicle to protect a vehicle occupant from coming into direct contact with a steering wheel and/or a windshield. The control system comprises a control circuit including a microcomputer. The microcomputer is adapted to execute the following control. A decision is made to operate (inflate) the airbag when a vehicle deceleration exceeds a predetermined threshold level. The threshold level is changed at a timing which is set in accordance with a physical amount representative of a deceleration condition of the vehicle, thereby securely operating the airbag merely by making a simple adjustment to the control system.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to improvements in a control system for anoccupant restraint system arranged to protect a vehicle occupant at avehicle collision.

2. Description of the Prior Art

A variety of control systems for an occupant restraint system have beenproposed and put into practical use in the field of automotive vehicle.The occupant restraint system includes an airbag and/or a seat beltassembly to protect a vehicle occupant from coming into direct contactwith a steering wheel and/or a windshield. One type of control systemsis arranged to operate the occupant restraint system to protect thevehicle occupant when a deceleration detected by a deceleration sensorexceeds a predetermined threshold level. Another type of control systemis arranged to operate the occupant restraint system when an integratedvalue reaches a predetermined threshold level, the integrated valuebeing obtained by integrating a value obtained by subtracting anintegration offset from a deceleration of the vehicle, as disclosed, forexample, in Japanese Patent Provisional Publication No. 63-503531.

However, the above conventional control systems require delicateadjustment of the threshold level and/or the integration offset inaccordance with a variety of modes of vehicle collision. This makes theadjustment of the control system complicated, requiring a long time forthe adjustment.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved controlsystem for an occupant restraint system, which can overcome drawbacksencountered in conventional control systems for an occupant restraintsystem.

Another object of the present invention is to provide an improvedcontrol system for an occupant restraint system, which can preciselyoperate the occupant restraint system to protect a vehicle occupant withrespect to a variety of modes of vehicle collisions, merely by making asimple adjustment to the control system.

An aspect of the present invention resides in a control system for anoccupant restraint system mounted on a vehicle, as illustrated inprinciple in FIG. 1A. The control system comprises first or decelerationdetecting means 100 for detecting a deceleration of the vehicle. Secondor operation deciding means 101 is provided to make a decision tooperate the occupant restraint system when the deceleration exceeds apredetermined threshold level. Third or physical amount calculatingmeans 102 is provided to calculate a physical amount representative of adeceleration condition of the vehicle. Fourth or changing timing settingmeans 103 is provided to set a changing timing am which the thresholdlevel is changed, in accordance with the physical amount. Fifth or levelchanging means 104 is adapted to change the threshold level at thechanging timing.

With this arrangement, the changing timing at which the threshold levelis changed is set in accordance with the physical amount representativeof the vehicle deceleration condition. The threshold level is changed atthe set timing, and the operation of the occupant restraint system isdecided when the vehicle deceleration exceeds the threshold level whichis before or after changing. Accordingly, the occupant restraint systemcan be precisely operated to protect a vehicle occupant with respect toa variety of modes of vehicle collision, merely by making a simpleadjustment to the control system.

Another aspect of the present invention resides in a control system foran occupant restraint system mounted on a vehicle, as illustrated inprinciple in FIG. 1B. The control system comprises first or decelerationdetecting means 200 for detecting a deceleration of the vehicle. Secondor integrating means 201 is provided to integrate a value obtained bysubtracting a predetermined integration offset from the deceleration, toobtain an integrated value. Third or operation deciding means 202 isprovided to make a decision to operate the occupant restraint system,when the integrated value exceeds a predetermined threshold level.Fourth or physical amount calculating means 203 is provided to calculatea physical amount representative of a deceleration condition of thevehicle. Fifth or changing timing setting means 204 is provided to set achanging timing at which the integration offset is changed, inaccordance with the physical amount. Additionally, sixth or integrationoffset changing means 205 is provided to change the integration offsetat the changing timing.

With this arrangement, the changing timing at which the integrationoffset is changed is set in accordance with the physical amountrepresentative of the vehicle deceleration condition. The integrationoffset is changed at the changing timing. Additionally, integration ismade on the value obtained by subtracting the integration offset (whichis before or after changing) from the vehicle deceleration to obtain theintegrated value. The decision to operate the occupant restraint systemis made when the integrated value exceeds the predetermined thresholdlevel. Accordingly, the occupant restraint system can be preciselyoperated to protect the vehicle occupant with respect to a variety ofmodes of vehicle collision, merely by making a simple adjustment to thecontrol system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram showing the principle of an aspect of thepresent invention;

FIG. 1B is a block diagram showing the principle of another aspect ofthe present invention;

FIG. 2 is a block diagram of a control system of the present invention,for an occupant restraint system;

FIG. 3 is a flowchart showing a main program of a control executed by amicrocomputer in the control system of FIG. 2, illustrating a firstembodiment of the control system of the present invention;

FIG. 4 is a flowchart showing a routine of calculating a lapsed timefrom the initiation of a vehicle collision, in the main program of FIG.3;

FIG. 5 is a flowchart showing a subroutine of Processing 1 in the mainprogram of FIG. 3;

FIG. 6 is a flowchart showing a subroutine of Processing 2 in the mainprogram of FIG. 3;

FIG. 7 is a graph showing variations in deceleration G for two vehiclecollision modes;

FIG. 8 is a graph showing variations in deceleration integrated value SGfor the two vehicle collision modes;

FIG. 9 is a flowchart showing a subroutine of Processing 1 in the mainprogram of FIG. 3, but illustrating a second embodiment of the controlsystem of the present invention;

FIG. 10 is a graph similar to FIG. 7 but showing the similar variationsin the second embodiment;

FIG. 11 is a flowchart showing a subroutine of Processing 1 in the mainprogram of FIG. 3, but illustrating a third embodiment of the controlsystem of the present invention;

FIG. 12 is a graph similar to FIG. 7 but showing similar variations inthe third embodiment;

FIG. 13 is a graph showing variations in deceleration differentiatedvalue DG for two vehicle collision modes in connection with the thirdembodiment;

FIG. 14 is a flowchart showing a main program of a control executed by amicrocomputer in a fourth embodiment of a control system according tothe present invention;

FIG. 15 is a flowchart showing a subroutine of Processing 1 in the mainprogram of FIG. 14;

FIG. 16 is a flowchart showing a subroutine of Processing 2 in the mainprogram of FIG. 14;

FIG. 17 is a graph showing variations in deceleration G for the twovehicle collision modes in connection with the fourth embodiment; and

FIG. 18 is a graph showing variations in deceleration integrated valueSG for the two vehicle collision modes in connection with the fourthembodiment.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIGS. 2 to 7, more specifically FIG. 2, of thedrawings, a first embodiment of a control system C of the presentinvention forms part of an occupant restraint system R for protecting avehicle occupant(s) from coming into direct contact with either asteering wheel, a windshield and/or the like upon a vehicle collision orthe like.

An arrangement of the first embodiment control system C of the presentinvention is shown in FIG. 2. The control system C comprises adeceleration sensor 10 which is adapted to detect a deceleration G of anautomotive vehicle (not shown) on which the occupant restraint system Ris mounted, and to output a signal representative of the deceleration Gto a control circuit 20. The control circuit 20 includes a microcomputerand its peripherals though not shown, and adapted to control operationof an airbag module 30 upon execution of a control program which will bediscussed hereinafter. In this embodiment, an airbag module 30 is storedin a central pad of a steering wheel of an automotive vehicle though notshown, so that the airbag module 30 protects a driver on a driver'sseat. The airbag module 30 includes an airbag which can inflate anddevelop to protect a vehicle occupant from coming into direct contactwith the steering wheel, the windshield and/or the like upon a vehiclecollision. The airbag module 30 further includes an inflator (not shown)for causing the airbag to inflate, and a squib 31 or electrical firingdevice for the inflator. An operating circuit 40 is electricallyconnected between the squib 31 and an electric source or battery 50 sothat electric current can be supplied from the electric source 50 to thesquib 31. Additionally, the operating circuit 40 is electricallyconnected to the control circuit 20. The electric source 50 iselectrically connected also to the control circuit 20.

The microcomputer of the control circuit 20 executes the control of amain program shown as the form of a flowchart in FIG. 3. The operationof the first embodiment control system C will be discussed withreference to the flowchart of FIG. 3.

At a step S1, a deceleration G of the vehicle is detected by thedeceleration sensor 10. Then, a flow goes to a step S2 to execute aroutine of calculating a lapsed time from the initiation of a vehiclecollision as shown in FIG. 4.

In the routine of FIG. 4, at a step S11 the detected deceleration G issubjected to a lowpass filter processing using a lowpass filter (LPF)thus obtaining a deceleration G'. At a subsequent step S12, a decisionis made as to whether the deceleration G' is larger than, for example, 1(G) or not. If larger, the flow goes to a step S13. If not larger, theflow goes to a step S14. At the step S13, 1 is set at a flag FT. At thestep S14, zero (0) is set at a lapsed time tcc (from the initiation ofthe vehicle collision) and at the flag FT. At a subsequent step S15, theflag FT is added to the lapsed time tcc (from the initiation of thevehicle collision) thereby to renew the lapsed time tcc, and then theflow returns to the main program of FIG. 3.

Upon returning to the main program, at a step S3, a decision is made asto whether a flag Fcon is zero (0) or not. If zero, the flow goes to astep S4. If not zero, the flow skips over the step S4 and goes to a stepS5. At the step S4, a subroutine of Processing 1 shown in FIG. 5 isperformed thereby to decide a time point (timing) TC at which athreshold level THL of the deceleration G is changed, in accordance withan integrated value SG of the deceleration G at a time point at which apredetermined time Maxt has lapsed from the initiation of the vehiclecollision.

In the subroutine of FIG. 5, at a step S21, a deceleration G which hasbeen detected at the current time (such as the current computercomputing cycle) is added to a deceleration integrated value SG obtainedup to a prior time (such as the immediately preceding computercomputation cycle) thereby obtaining a new deceleration integrated valueSG. At a subsequent step S22, a decision is made as to whether thelapsed time tcc exceeded the predetermined time Maxt or not. If exceeds,the flow goes to a step S23. If not exceeds, the flow returns to themain program. At the step S23, 1 is set at the flag Fcon, and then theflow goes to a step S24 at which a ratio ret between the decelerationintegrated value SG and a predetermined value THS is obtained. At a stepS25, a predetermined time TC for changing the deceleration thresholdlevel THL is multiplied by the ratio ret so as to be changed. It is tobe noted that the time point (TC+Maxt) obtained by adding thepredetermined lapsed time Maxt (from the vehicle collision initiation)to the predetermined changing time point TC after being changedcorresponds to a time point (timing) at which the threshold level THL ischanged on a standard or the time point of the vehicle collisioninitiation. The threshold level THL is a standard value for deciding asto whether the airbag is operated (inflated) or not. In this embodiment,when the deceleration G exceeds the threshold level THL, a decision ismade to operate (inflate) the airbag. When the above processing iscompleted, the flow returns to the main program of FIG. 3.

After returning to the main program, an a step S5, a subroutine ofProcessing 2 shown in FIG. 6 is performed to make a decision as towhether the airbag is operated (inflated) or not upon comparison of thedeceleration G with a threshold level THG1 or THG2.

More specifically, at a step S31, a decision is made as to whether thechanging time point (TC+Maxt) for the threshold level THL has beenreached or not. If reached, the flow goes to a step S32. If not reached,the flow goes co a step S34. At the step S32, a decision is made as cowhether the deceleration G is larger than the threshold level THG2 ornot. If larger, the flow goes to a step S33. If not larger, the flowreturns to the main program of FIG. 3. At the step S34 or before thechanging time point for the threshold level THL, a decision is made asto whether the deceleration G is larger than the threshold level THG1 ornot. If larger, the flow goes to a step S33. If not larger, the flowreturns to the main program. Here, the threshold levels THG1, THG2 areset to be in a relationship of THG1>THG2. At the step S33, since thedeceleration exceeds the threshold level THG1 or THG2, a decision ismade to operate or inflate the airbag, so that the control circuit 20outputs an operating signal to the operating circuit 40. As a result,the operating circuit 40 causes electric current to be supplied from theelectric source 50 to the squib 31. Then, the squib 31 fires theinflator thereby inflating and developing the airbag.

Hereinafter, advantageous effects of the first embodiment control systemC will be discussed with reference to FIGS. 7 and 8. FIG. 7 depictsvariations in the deceleration G during a vehicle collision. FIG. 8depicts variations in the integrated value SG of the deceleration G.

Explanation will be made, for example, on two types of vehiclecollisions. One of them is a case in which the deceleration G relativelyquickly increases immediately after the vehicle collision and thenquickly decreases, in which the impact of the collision is relativelysmall and therefore no operation (inflation) of the airbag is necessary.Such type of vehicle collision is referred hereinafter to as a "lightcollision". The other of them is a case in which the deceleration Grelatively gradually increases immediately after the vehicle collisionand thereafter largely increases, in which the impact of the collisionis large and therefore the operation (inflation) of the airbag isnecessary to securely protect the vehicle occupant. Such type of vehiclecollision is referred hereinafter to as a "low speed collision".

As apparent from FIG. 8, the ratio ret at the time point at which thetime Maxt has lapsed from the initiation of the vehicle collision issmaller in the low speed collision than that in the light collision, sothat the changing time point TC in the low speed collision is advancedrelative to that in the light speed collision. Accordingly, thethreshold level THL in the low speed collision is changed early ascompared with that in the light speed collision. As shown in FIG. 7, incase of the light collision, it is judged or decided that no operation(inflation) of the airbag is necessary, because the deceleration G islower than the threshold level THG2 even upon changing the thresholdlevel from THG1 to THG2. In case of the low speed collision, it isjudged or decided to operate (inflate) the airbag, because thedeceleration G exceeds the threshold level THG2 upon changing thethreshold level from THG1 to THG2.

Thus, the first embodiment control system is arranged such that the timepoint at which the threshold level THL of the deceleration G is changedis set in accordance with the integrated value of the deceleration Grepresentative of a deceleration condition of the vehicle; and theairbag is operated when the deceleration G exceeds the threshold levelTHL at its state before being changed or after being changed. Therefore,the airbag can be precisely and securely operated (inflated) withrespect to a variety of vehicle collisions merely upon making simpleadjustment to the control system.

FIGS. 9 and 10 illustrate a second embodiment of the control system C inaccordance with the present invention. The second embodiment controlsystem C is similar to the first embodiment control system C with theexception that the time point at which the threshold level THL of thedeceleration G is changed is set in accordance with the deceleration G.In this connection, the same timing is set in accordance with thedeceleration integrated value SG in the first embodiment control systemC. Accordingly, the arrangement of the second embodiment control systemC is the same as that of the first embodiment control system C shown inFIG. 2, and therefore explanation thereof is omitted for the purpose ofsimplicity of illustration. Additionally, a control program of thesecond embodiment control system C is the same as that of the firstembodiment control system C except for the subroutine of Processing 1,and therefore the control programs shown in FIGS. 3, 4 and 6 are commonin the control program of the second embodiment. In view of this,discussion of the second embodiment control system will be made mainlyaccording to a subroutine of Processing 1 shown in FIG. 9 whichsubroutine is different from that of the first embodiment control systemC shown in FIG. 5.

FIG. 9 depicts the subroutine of Processing 1 of the second embodimentcontrol system. The microcomputer of the control circuit 20 executes thesubroutine of FIG. 9 thereby to decide a time point TC at which thethreshold level THL is changed, in accordance with the deceleration G ata time point (timing) at which the time Maxt has lapsed from theinitiation of the vehicle collision.

More specifically, at a step S41, a decision is made as to whether thetime Maxt has lapsed from the initiation of the vehicle collision ornot. If lapsed, the flow goes to a step S42. If not lapsed, the flowreturns to the main program shown in FIG. 3. At a step S42, 1 is set atthe flag Fcon, and then the flow goes to a step S43 at which the ratioret between the deceleration G and the predetermined value THG isobtained. At a step S44, the predetermined time point TC for changingthe threshold level THL is multiplied by the ratio ret, so that the timepoint is changed. When the above processing is completed, the flowreturns to the main program of FIG. 3.

Advantageous effects of the second embodiment control system will bediscussed with reference to FIG. 10 which depicts variations indeceleration G with respect to the light collision and the slow speedcollision.

As apparent from FIG. 10, the ratio ret at the time point at which thetime Maxt has lapsed from the initiation of the vehicle collision issmaller in the low speed collision than that in the light collision, sothat the changing time point TC in the low speed collision is advancedrelative to that in the light speed collision. Accordingly, thethreshold level THL in the low speed collision is changed early ascompared with that in the light speed collision. In case of the lightcollision, it is judged or decided that no operation (inflation) of theairbag is necessary, because the deceleration G is lower than thethreshold level THG2 even upon changing the threshold level from THG1 toTHG2. In case of the low speed collision, it is judged or decided tooperate (inflate) the airbag, because the deceleration G exceeds thethreshold level THG2 upon changing the threshold level from THG1 toTHG2.

Thus, the second embodiment control system is arranged such that thetime point at which the threshold level THL of the deceleration G ischanged is set in accordance with the deceleration G representative of adeceleration condition of the vehicle; and the airbag is operated whenthe deceleration G exceeds the threshold level THL in its state beforebeing changed or after being changed. Therefore, the airbag can beprecisely and securely operated (inflated) with respect to a variety ofvehicle collisions merely upon making simple adjustment to the controlsystem. This embodiment can omit the integrating processing of thedeceleration G made in the first embodiment control system, therebylightening the load of the microcomputer as compared with the firstembodiment control system C.

FIGS. 11, 12 and 13 illustrate a third embodiment of the control systemin accordance with the present invention. The third embodiment controlsystem is similar to the first embodiment control system C with theexception that the time point at which the threshold level THL of thedeceleration G is changed is set in accordance with a differentiatedvalue DG of the deceleration G. Accordingly, the arrangement of thesecond embodiment control system C is the same as that of the firstembodiment control system C shown in FIG. 2, and therefore explanationthereof is omitted for the purpose of simplicity of illustration.Additionally, a control program of the third embodiment control system Cis the same as that of the first embodiment control system C except forthe subroutine of Processing 1, and therefore the control programs shownin FIGS. 3, 4 and 6 are common in the control program of the thirdembodiment. In view of this, discussion of the third embodiment controlsystem will be made mainly according to a subroutine of Processing 1shown in FIG. 11 which subroutine is different from that of the firstembodiment control system C shown in FIG. 5.

FIG. 11 depicts the subroutine of Processing 1 of the second embodimentcontrol system. The microcomputer of the control circuit 20 executes thesubroutine of FIG. 11 thereby to decide a time point (timing) TC atwhich the threshold level THL is changed, in accordance with thedifferentiated value DG of the deceleration G at a time point (timing)at which the time Maxt has lapsed from the initiation of the vehiclecollision.

More specifically, at a step S51, a deceleration GB detected at theprior time (such as the immediately preceding computer computationcycle) is subtracted from the deceleration G detected at the currenttime (such as the current computer computing cycle) to obtain adifference in deceleration between the prior time and the current time,i.e., a differentiated value DG of the deceleration G. At a subsequentstep S52, the deceleration G detected at the current time is set for thedeceleration GB, and then the flow goes to a step S53. At the step S53,a decision is made as to whether the lapsed time tcc exceeds the timeMaxt or not. If exceeded, the flow goes to a step S54. If not exceededthe flow returns to the main program of FIG. 3. At the step S54, 1 isset at the flag Fcon, and then the flow goes to a step S55 in which adecision is made as to whether the deceleration differentiated value DGis positive or nom, i.e., the deceleration G is increasing or not. Ifthe deceleration G is increasing, the flow goes to a step S56. If notincreasing, the flow goes to a step S57. At the step S56, 0 is set forthe changing time point TC for the threshold level THL. At the step S57,a predetermined value is set, as it is, for the changing time point TC,and then the flow returns to the main program.

Advantageous effects of the third embodiment control system will bediscussed with reference to FIG. 12 which shows variations indeceleration G with respect to the light collision and the low speedcollision and to FIG. 13 which shows variations in the differentiatedvalue DG of the deceleration G in FIG. 12.

As shown in FIG. 13, the differentiated value DG of the deceleration Gat the time point at which the time Maxt has lapsed from the initiationof the vehicle collision is negative in case of the light collision andpositive in the case of the low speed collision. Accordingly, thethreshold level THL in the low speed collision is changed early ascompared with that in the light speed collision. In case of the lightcollision, it is judged or decided that no operation (inflation) of theairbag is necessary, because the deceleration G is lower than thethreshold level THG2 even upon changing the threshold level from THG1 toTHG2. In case of the low speed collision, it is judged or decided tooperate (inflate) the airbag, because the deceleration G exceeds thethreshold level THG2 upon changing the threshold level from THG1 toTHG2.

Thus, the third embodiment control system is arranged such that the timepoint at which the threshold level THL of the deceleration G is changedis set in accordance with the differentiated value of the deceleration Grepresentative of a deceleration condition of the vehicle; and theairbag is operated when the deceleration G exceeds the threshold levelTHL in its state before being changed or after being changed. Therefore,the airbag can be precisely and securely operated (inflated) withrespect to a variety of vehicle collisions merely upon making simpleadjustment to the control system. Additionally, the differentiatingprocessing of the deceleration G is simple in calculation as comparedwith the integrating processing of the same, thereby lightening the loadof the microcomputer as compared with the first embodiment controlsystem C.

FIGS. 14 to 18 illustrate a fourth embodiment of the control system Caccording to the present invention, in which the time point at which anintegration offset (a changeable value for setting a base point orstandard for integration) of the deceleration G is changed is set inaccordance with the integrated value SG of the deceleration G;integration is made on a value obtained by subtracting the integrationoffset at its state before being changed or after being changed, fromthe detected deceleration G; and the airbag is operated when thedeceleration integrated value SG exceeds a threshold level.

The arrangement of the fourth embodiment control system is the same asthat of the first embodiment control system C shown in FIG. 2, andtherefore explanation thereof is omitted for the purpose of simplicityof illustration.

In this embodiment, the microcomputer of the control circuit 20 of thecontrol system C executes the control of a main program shown in FIG.14. The operation of the control system C of this embodiment will bediscussed with reference to a flowchart of FIG. 14.

At a step S61, the deceleration G of the vehicle is detected by thedeceleration sensor 10. At a subsequent step S62, the subroutine shownin FIG. 4 is performed thereby to obtain the lapsed time tcc (from theinitiation of the vehicle collision). At a step S63, an integrationoffset OFF is subtracted from the deceleration G detected at the currenttime (such as the current computer computation cycle) to obtain a value.This value is added to the integrated value SG obtained up to the priortime (such as the immediately preceding computer computation cycle) thusobtaining an integrated value SG of the deceleration G. Subsequently, ata step S64, a decision is made as to whether the flag Fcon is at zero(0) or not. If at zero, a flow goes to a step S65. If not at zero, theflow skips over the step S66. At the step S65, a subroutine ofProcessing 1 shown in FIG. 15 is performed in which a time point(timing) TC at which the integration offset OFF is changed is decided inaccordance with the deceleration integrated value SG.

In the subroutine of FIG. 15, at a step S71, a decision is made as towhether the lapsed time tcc (from the initiation of the vehiclecollision) has exceeded a predetermined time Maxt. If exceeded, the flowgoes to a step S72. If not exceeded, the flow returns to the mainprogram of FIG. 14. At a step S72, 1 is set at the flag Fcon, and thenthe flow goes to a step S73 at which the deceleration integrated valueSG is divided by a predetermined value THS thereby to obtain a ratioret. At a step S74, the previously set time point (timing) TC at whichthe integration offset OFF is changed is multiplied by the ratio ret sothat the time point is changed. Then, the flow returns to the mainprogram.

Upon returning to the main program of FIG. 14, at a step S66, asubroutine of Processing 2 shown in FIG. 16 is performed in which theintegration offset OFF is changed; and a decision is made as to whetherthe airbag is operated (inflated) or not in accordance with a previouslyset threshold level THL/S.

More specifically, at a step S81 in the subroutine of FIG. 16, adecision is made as to whether the time point (TC+Maxt) at which theintegration offset OFF is changed has been reached or not. If reached,the flow goes to a step S82 at which a value OFF2 is set for theintegration offset OFF. If not reached, the flow goes to a step S83 atwhich a value OFF1 is set at the integration offset OFF. Here, a settingis made between the values OFF1 and OFF2 to establish the relationshipof OFF1>OFF2. At a step S84, a decision is made as to whether theintegrated value SG of the deceleration G exceeds the threshold levelTHL/S or not. If exceeds, the flow goes to a step S85. If not exceeded,the flow returns to the main program. At the step S85, a decision ismade to operate (inflate) the airbag, and therefore the control circuit20 outputs the operating signal to the operating circuit 40.

Advantageous effects of the fourth embodiment control system C will bediscussed with reference to FIGS. 17 and 18. FIG. 17 depicts variationsin the deceleration G with respect to the light collision and the lowspeed collision. FIG. 18 depicts variations in the decelerationintegrated value SG with respect to the two types of collisions of FIG.17.

As apparent from FIG. 18, the ratio ret at the time point at which thetime Maxt has lapsed from the initiation of the vehicle collision issmaller in the low speed collision than that in the light collision, sothat the changing time point TC in the low speed collision is earlierthan that in the light speed collision. Accordingly, the integrationoffset OFF is changed earlier in the low speed collision than that inthe light collision.

Here, as shown in FIG. 17, assuming that the integration offset OFF2 iszero (0), the integration offset becomes zero earlier in the low speedcollision than that in the light collision. Therefore, in case of thelow speed collision, the deceleration integrated value SG rapidlyincreases when the time lapses over the changing time point (TC+Maxt)for the integration offset, and reaches the previously set thresholdlevel THL/S of the deceleration integrated value SG thereby making thedecision to operate (inflate) the airbag. In case of the lightcollision, the integration offset becomes zero later than that in thelow speed collision; however, the deceleration G has already rapidlydecreased at that time point (timing) and therefore the decelerationintegrated value SG does not increase. Accordingly, the decelerationintegrated value does not reach the threshold level THL/S, so that theairbag cannot be operated (inflated).

Thus, the fourth embodiment control system is arranged such chat thetime point at which the integration offset of the deceleration G ischanged is set in accordance with the integrated value SG of thedeceleration G representative of a deceleration condition of thevehicle; integration is made on the value obtained by subtracting theintegration offset at its state before being changed or after beingchanged, from the detected deceleration G; and the airbag is operatedwhen the deceleration integrated value SG exceeds the threshold levelTHL/S. Therefore, the airbag can be precisely and securely operated(inflated) with respect to a variety of vehicle collisions merely uponmaking simple adjustments to the control system.

While only the occupant restraint system R including the airbag for adriver has been shown and described as being controlled by the controlsystem C, it will be understood that the principle of the presentinvention may be applicable to other occupant restraint systemsincluding airbag and/or seat belt and to those systems for protectingvehicle occupants on a front seat located aside the driver's seat and ona rear seat.

What is claimed is:
 1. A control system for an occupant restraint systemmounted on a vehicle, comprising:means for detecting a deceleration ofthe vehicle; means for making a decision to operate the occupantrestraint system when the deceleration exceeds a predetermined thresholdlevel; means for calculating a physical amount representative of adeceleration condition of the vehicle; means for setting a changingtiming at which the threshold level is changed, in accordance with thephysical amount; and means for changing the threshold level at thechanging timing.
 2. A control system as claimed in claim 1, wherein saidphysical amount determines the deceleration detected by saiddeceleration detecting means.
 3. A control system as claimed in claim 2,wherein said physical amount is the deceleration detected by saiddeceleration detecting means.
 4. A control system as claimed in claim 2,wherein said physical amount is an integrated value of the decelerationdetected by said deceleration detecting means.
 5. A control system asclaimed in claim 2, wherein said physical amount is a differentiatedvalue of the deceleration detected by said deceleration detecting means.